and you wish to use it as the key for an unordered_map.
You need to customise the hash for this structure. To do this we need to combine
the hash values for x and
y. The function boost::hash_combine is supplied for
this purpose:

Calls to hash_combine incrementally build the hash from the different members
of point, it can be repeatedly called for any number of elements. It calls
hash_value on the supplied
element, and combines it with the seed.